1. Field of the Invention
The present invention relates to a technical field of an aberration correcting apparatus for correcting a wavefront aberration (mainly a coma aberration) generated on an information recording surface of an information record medium for an optical information reproduction due to an inclination of an angle formed between the information recording surface and an optical axis of an optical beam for the information reproduction from the right angle. (Hereafter, this inclination is referred to as a tilt, and the amount of this inclination is referred to as a tilt amount.)
2. Description of the Related Art
In such an aberration correcting device, the wavefront aberration caused, for example, in the direction of a radius of an optical disc functioning as an information record medium (hereafter referred to as radial direction) due to the tilt is corrected by: emitting an optical beam for a tilt amount detection onto the optical disc besides the above described optical beam for the information reproduction; receiving the reflected light of the optical beam for the tilt amount detection by using a photodetector divided into two partial detection portions by a division line perpendicular to the radial direction (hereafter, the emitting device for emitting the optical beam for the tilt amount detection and the photodetector are referred to collectively as a tilt sensor); calculating the above described tilt amount from the difference between the detected signals supplied from the respective partial detection portions; and inclining the optical axis of the optical beam for the information reproduction so as to cancel the tilt on the basis of the calculated tilt amount.
In this aberration correcting apparatus, however, it is necessary to cancel the tilt by inclining the optical axis by using a mechanical drive device. Therefore, the aberration correcting apparatus has problems of an increased manufacturing cost, a lowered reliability and a difficulty in a size reduction. These problems occur for the tilt in the rotation direction of the optical disc (hereafter, which referred to as a tangential direction) as well in the same way.
Therefore, as an improvement of the above described mechanical aberration correcting apparatus, there is a method of giving a phase difference or the like to the optical beam for the information reproduction on the basis of the detected tilt amount without using the mechanical tilt canceling device, to thereby optically cancel the wavefront aberration caused by the tilt.
However, according to the above described method of optically canceling the aberration, the tilt itself is not canceled but the wavefront aberration caused by the tilt is canceled. Thus, even if the wavefront aberration is canceled, the tilt itself is not changed. As a result, the detected signal supplied from the tilt sensor is not changed, either.
Therefore, it cannot be determined from the detected signal supplied from the tilt sensor whether the optical correction is insufficient or superfluous. Unlike the above described mechanical aberration correcting apparatus, therefore, a so-called feedback control based on the detected signal of the tilt sensor cannot be conducted. Instead, a so-called feedforward control of estimating the wavefront aberration (i.e., the tilt amount) on the basis of the detected signal supplied from the tilt sensor and conducting the correction is thus execution.
Therefore, if the tilt amount cannot be detected accurately by the tilt sensor, the aberration correction cannot be conducted properly. Thus, there is a problem that the dispersion of sensitivities and offsets or nonlinear characteristics of the partial detection portions in the tilt sensor must be adjusted accurately.
Furthermore, there is a problem that a change is caused in some cases by an elapse of time even or aging even if these values are adjusted accurately in the initial state.
Furthermore, there is a problem that, in the case where both of the tilt in the radial tilt direction and the tilt in the tangential direction are to be corrected, two tilt sensors must be used resulting in a large sized pickup portion.